Traffic Radar: Introduction to Speed Enforcement

Introduction to Traffic Radar

• RADAR stands for Radio Detecting and Ranging, a device used to detect and measure the speed of distant objects by analyzing radio waves reflected from their surfaces.
• The first traffic law was passed in 1652, prohibiting the riding or driving of horses at a gallop within city limits.
• The first automobile speed regulation was enacted in 1912, limiting automobile speeds to 12 mph in the country and 8 mph within city limits.

Frequency and Wave Length

• Frequency is the number of waves transmitted in one second, usually measured in cycles per second.
• Wavelength is the distance from the beginning of the peak to the end of the valley, with most radars operating within the X-band (10.525 billion waves per second) or K-band (24.15 billion waves per second).
• Radar signals travel at the speed of light, 186,000 miles per second.

Doppler Principle

• The Doppler Principle states that relative motion causes a signal's frequency to change.
• When objects are moving closer together, the frequency increases, and when they are moving farther apart, the frequency decreases.
• The amount of frequency change is determined by the speed of the relative motion.

Factors Affecting Radar Accuracy

• Angle Effect: The angle of the radar antenna can cause an improper target reading.
• Reflective capability: The radar unit's operation is affected by the reflective capability of targets, their position, and their actual speed.
• Batching: The radar adds a fraction of the patrol vehicle's speed to the violator's speed.
• Scanning Effect: The rapid swinging past a fixed object of a stationary radar antenna can produce a speed reading.
• Panning Effect: Can occur with two antenna units when the antennas are pointed at each other, causing feedback.
• Turn on Power Surge: Turning on the power to a unit can cause a speed reading due to the sudden surge of voltage.
• Mirror Switching Effect: Can occur on Speed gun Eights if the operator points the unit out the back window and uses the mirror to read the display without activating the mirror switching device.
• Patrol Speed Shadow Effect: Occurs if the beam of the radar unit attaches onto a moving object instead of the terrain.

Basic Operations of Moving Radar

• Target Speed = Closing Speed - Patrol Speed
• High Doppler and Low Doppler frequencies are used to calculate the target speed.
• Testing the unit involves activating the high and low tuning forks and holding them in front of the unit to give a target speed.

Fundamentals of Case Law

• The officer must prove that a particular speed law was violated, the defendant was the driver, and the offense occurred on a public thoroughfare.
• The officer must also prove that the correct vehicle was stopped, and that the vehicle in question appeared to be traveling at the speed indicated on the RADAR equipment.
• Special requirements for moving radar include:
  • Operator having sufficient training and experience
  • The radar instrument must have been in proper working condition (internal test)
  • The device was used where road conditions would distort the readings as little as possible (e.g., not raining)
  • The patrol car's speed was verified (speedometer)
  • The instrument's accuracy was tested (tuning forks)